1. Field of the Invention
The present invention relates to a battery charger and also to a charging method of secondary batteries, such as nickel-cadmium batteries.
2. Description of the Related Art
It is essential for battery chargers to determine during a charging process that a battery has reached a fully charged condition. One method of determining the fully charged condition is a "-.DELTA.V detection method", in which the battery is determined to be fully charged when the battery voltage drops a predetermined voltage (.DELTA.V) from the peak level. To this end, the voltage Vi across the battery is detected at every predetermined sampling interval, and whenever the detected battery voltage Vi exceeds the ever occurring maximum voltage, the data recorded as the maximum voltage is updated. When the battery voltage Vi drops a predetermined voltage from the maximum voltage Vmax, the battery is determined to be fully charged. See FIG. 3. Hereinafter, detection of a point where the battery voltage drops a predetermined voltage from the maximum voltage will hereinafter be referred to as "-.DELTA.V detections".
Batteries subject to charging with the charger can be classified into some categories in terms of the use circumstance and residual amount of charges. Specifically, according to one aspect, batteries can be classified into charged ones and discharged ones. Typically, it is a discharged battery that is loaded into the charger for charging. However, fully charged batteries may erroneously be loaded into the charger when the user does not know that the battery has already been fully charged. According to another aspect, the batteries can be classified into active batteries and inactive batteries. Batteries that are normally discharged through the use with electrically powered products are referred to as active batteries. Inactive batteries are such batteries that have been left unused for a long period of time, causing the battery to discharge.
When inactive batteries are charged, the voltage across the battery first drops at the initial stage of charging as shown in FIG. 4. When the -.DELTA.V detection method is applied to the inactive batteries, detection of the fully charged condition cannot be performed correctly because the -.DELTA.V detection method detects the voltage drop appearing at the initial stage of charging and determines that the battery is fully charged at this point. In order to solve this problem, a modified -.DELTA.V detection method has been proposed in which the -.DELTA.V detection is not performed during a predetermined period of time from the start of charging to thus disregard the firstly appearing peak point.
However, this modified -.DELTA.V detection method invites undesirable results when charging a fully charged battery. Notwithstanding the fact that the fully charged battery exhibits a peak to be detected at the initial stage of charging, it is disregarded. As a result, the battery is overcharged and at worst the battery is damaged.
To obviate the problem accompanying the modified -.DELTA.V detection method, an improved -.DELTA.V detection method has been proposed in which a status of the battery is determined based on the battery voltage detected through a provisional charge of the battery and a time duration at which the -.DELTA.V detection is not performed is determined depending upon the status of the battery. However, this improvement is still unsatisfactory in the following respects. First, determination of the status of the battery is complicated. Second, in the case of charging batteries having different number of cells, the status determination may not be performed accurately when each cell does not have an equal capacitance or when one or more of the cells are short-circuited.